I love your question - I just ordered the book hand talk and will get back to you after I read it! I'm expecting some answers there.

That said - I know that it did diverge from region to region, but was still intelligible, and I also know that all languages change over time, though sometimes lingua francas change a bit less. They still change though.

The question of 'how much' is what I'm hoping to find out from the book.